1. Field of the Invention
The present invention relates to glycopeptides which are amphipathic. The glycopeptides of the present invention are capable of crossing the blood-brain-barrier (BBB). As a result, the glycopeptides for treating a variety of neurological and behavioral disorders.
2. Description of the Background
Endogenous opioid peptides, lumped together under the generic term endorphins, have been the subject of intense study since their discovery in the mid 1970's.1 Neuropeptides have the potential for extremely selective pharmacological intervention with fewer side effects. If these naturally occurring opioid peptides and their derivatives could be rendered permeable to the blood-brain barrier (BBB), then a new vista of psychopharmacology would be opened to exploration. After three decades of research, many potent and selective opioid agonists have been developed, and stability problems have been largely overcome. The principal remaining problem that prevents the use of opioid peptides as drugs is poor bioavailability, which is due to poor penetration of the BBB.2 The BBB is composed of endothelial cells in the cerebrovascular capillary beds.3 The BBB acts as a barrier to undesired chemical substances, and admits vital nutrients for proper function of the CNS.4 The flow is bi-directional, allowing for export of materials from the CNS (efflux transport) and the import of materials from the blood (influx transport). The BBB represents not only a physical obstacle, but a metabolic one as well, possessing both oxidative enzymes and peptidases such as aminopeptidase, arylamidase and enkephalinase. Thus, metabolically unstable substances (e.g. peptides) are generally degraded before they reach the CNS. It should also be noted that entry to the CNS does not guarantee that a drug will accumulate in useful concentrations, as many peptides are rapidly exported back to the bloodstream.5 Several strategies have been reported to overcome the BBB penetration problem, including substitution of unnatural amino acids,6 the use of conformational constraints,7 and the addition of lipophilic side chains or other transport vectors.8 Glycosylation has proven to be a successful methodology to improve both the stability and bioavailability of short peptide “messages” by incorporation of the peptide pharmacophore into a glycopeptide.9 Previous BBB penetration studies with opioid glycopeptide agonists based on enkephalins have shown up to 3-fold increases in the rate of brain delivery of these compounds compared with the unglycosylated parent peptides.10 Recent studies with glycopeptides in artificial membrane systems indicate that amphipathicity of the glycopeptides is an important factor in BBB penetration.11 In addition, there is evidence that suggests that the type of glycosylation can alter tissue distribution patterns,12 BBB penetration,13 and peptide/receptor interactions.11,14 
Endogenous Opioid Peptides. The endogenous neuropeptide β-endorphin is a 31 residue naturally occurring opioid peptide agonist that binds to μ and δ receptors. Its N-terminal 5 residues are identical to the Met-Enkephalin sequence, and may be considered to be the pharmacophore. It was shown some time ago that the C-terminal region of β-endorphin has an amphipathic α-helical structure that plays a role in the receptor binding and opioid agonism,15 and may induce resistance to proteolysis.16 According to Schwyzer, the N-terminal sequence is the essential “message,” and the C-terminal helical region is the “address” that limits delivery of the message to a subset of otherwise available opioid receptors.17 Kaiser and co-workers proposed that β-endorphin consists of the Met-enkephalin peptide sequence at the N-terminus, a hydrophilic linker region from residues 6 through 12, and an amphiphilic helical region between the helix breaker residues Pro(13) and Gly(30).18 This was later proven by synthesizing a number of β-endorphin mimics with artificial C-terminal helical regions with amphipathic character.19 These de novo amphipathic helices were non-homologous to the β-endorphin C-terminal region, and they were shown to be largely α-helical by CD measurements. These hybrid structures showed good opioid agonism in vitro when compared to β-endorphin. These studies strongly suggested that the amphipathicity of the C-terminal helix plays a key role in the selectivity of these compounds, rather than the identity of specific amino-acids present in the C-terminal.20 Dynorphin A (1-17) is also an endogenous opioid peptide, but it binds preferentially to the κ opioid receptor and has an N-terminal message segment identical to Leu-Enkephalin.21 It has been suggested that an address sequence in the C-terminal region imparts selectivity for κ receptors.22 Dynorphin A adopts an extended and/or random coil structure when subjected to structural analysis by various spectroscopic measurements.23 A 2D 1H-NMR study in DPC micelle shows that Dynorphin A(1-17) contains a less ordered N-terminal segment, a well defined α-helix segment spanning between Phe(4) and Pro(10) or Lys(11), and a β-turn from Trp(14) to Gln(17).24 Based on NMR results, the authors concluded that both the α-helix and the C-terminal β-turn are due to dynorphin-micelle interactions, and may be important structural features of the full-length peptide when bound to the cell membrane in vivo. Studies by Luna25 also support the notion that a helical structure in the message segment of Dynorphin A(1-17) is significant. The biological importance of helical C-terminal address segments in larger opioid peptides has been further supported by the recent work by Kyle and co-workers.26 They successfully synthesized several potent nociceptin (NC) peptide analogs exploiting the α-helix-promoting residues α-aminoisobutyric acid (Aib) and N-methyl alanine (MeAla) at the C-terminus of NC. Nociceptin is the endogenous ligand for the recently identified opioid receptor-like 1 receptor (ORL-1). Thus, it seems logical to approach the design of opioid agonist β-endorphin or dynorphin peptide analogs by combining C-terminal amphipathic helical address segments that can also promote BBB penetration by virtue of glycosylation.